1. Field of the Invention
The present invention relates to a method for test call synchronization in a code division multiple access (CDMA) cellular system. More particularly, this invention relates to a method for test call synchronization between a mobile station and a base station without using Global Position System Time as absolute time.
2. Description of the Related Art
A cellular system requires extensive manpower and capital to carry out the network management function. A test call is provided as a solution to the problem by providing operators with means for measuring the composite quality of forward and reverse links in BSC (Base Station Controller). The test call does not measure a quality of one cell but rather measures the quality under environments including handoffs among a plurality of cells, as seen in FIG. 1 such as cells 100, 110, 120, and 130. The creation and comparing of packets are preferably performed in the TSB (Transceiver and Selector Bank 140) of the base station. FIG. 1 illustrates an example of a general TSB, which manages the test call of a CDMA cellular system in accordance with the prior art. As illustrated in FIG. 1, it is necessary for the TSB to tune the synchronization with a mobile station to compare and analyze random data of the test call.
An option of mobile station loop-back service makes it possible to obtain all sorts of the quality information, based on an initial traffic information passed through the mobile station. With the option, the TSB sends the mobile station a series of information already known by the TSB. The mobile station receives the information and measures the forward link quality by confirming whether the information has been normally received, and loops-back the received information to the TSB. The measurement result is transmitted as reverse information and then re-sends the received information on the traffic. The TSB is able to measure the transmit/receive quality of the mobile station, based on the received information. Also, the test call makes it possible to establish a call to measure the performance of the overall system or makes it convenient to generate traffic data.
In order for the TSB to make a quality measurement, it is necessary for the TSB to already know information from the loop-back by the mobile station, so it is necessary to tune the synchronization between the mobile station and transceiver. A conventional means of tuning synchronization is performed by comparing packets by maintaining the index of forward and reverse frames derived from the establishment of a test call to the call""s termination.
FIG. 2 shows a block diagram illustrating the conventional method for processing the transmit/receive index for test call synchronization. As illustrated in FIG. 2, the base station 204 increases a transmit-index, tx-index, whenever it transmits the frame 210 to the mobile station 203, in establishing test call 200. Base station 204 also increases a receive-index, rx-index, from the time when good frames are received sequentially as at 220. The conventional method for synchronization causes the reverse frame index to disperse, and as a result, the synchronization becomes de-tuned in the following cases:
1. First casexe2x80x94other frames are received by the TSB owing to differences of delay times because one mobile station receives radio waves from a plurality of base stations increasing the probability of occurrence of continuous handoffs in a congested radio wave environment typically associated with the center of a city;
2. Second casexe2x80x94when the reverse link is degraded and the quality of a series of sequential frames is damaged, the mobile station temporarily stops transmitting reverse frames and sequentially receives good forward frames for a specific time and then resumes transmitting the reverse frames; and
3. Third casexe2x80x94the reverse link is damaged owing to a specific cause regardless of the forward link quality and preventing the base station from receiving the reverse frames.
The above cases suffer from a decreased reliability of the measured result because there is no way to normalize the synchronization with the mobile station, and consequently, the quality of the link becomes disconnected with the measured result, even though the quality of reverse link returns is acceptable.
A similar method is addressed in U.S. Pat. No. 5,649,000, which discloses a method and apparatus for synchronizing timing among radio ports in wireless communications systems. In the method, time synchronization is achieved from the average time differences by receiving time from the frequencies with the highest quality. However, the method doesn""t measure an exact time but an approximate time.
The present invention, as embodied and broadly described herein, provides a method for test call synchronization of a code division multiple access (CDMA) cellular system. The method comprises the steps of: (a) establishing a test call for a mobile station and base station; (b) maintaining a transmit index and sending the transmit index on each forward frame sent to the mobile station, the transmit index being masked from forward frames forming part of an initial traffic sent to the mobile station, the value of the transmit index being increased for every forward frame transmitted; (c) measuring forward link quality by the mobile station for each forward frame received by the mobile station and sending a reverse frame from the mobile station to the base station including information on said measured forward link quality and information contained in the forward frame (d) receiving and checking said reverse frame by the base station, and if the result of checking is normal, setting the value of a receive index to be equal to that of said transmit index included in the reverse frame, otherwise increasing the value of said receive index; and (e) substituting the transmit-index for the receive-index when the mobile station receives each frame from the base station, and as a result, the reverse link is good and the transmit-index is exact, wherein the transmit index being on the reverse frame.
According to the method of the present invention, the steps are repeatedly performed at the receipt of each frame. The method maintains the synchronization without hardware reference time in a test call of CDMA cellular system. The TSB maintains the synchronization for a test call and the test call synchronization for the network management of CDMA cellular system is performed independently of absolute time.
The present invention is advantageous in that it achieves a reliable measurement by maintaining synchronization with a mobile station regardless of link quality, as it is possible to solve the problem of the conventional method in that the synchronization is inconsistent with each other due to the degradation of link which causes the reverse frame index to disperse.